Pyrethrines (or pyrethrum), that is esters of chrysanthemic acid (2,2-dimethyl-3-isobutenyl-cyclopropane-carboxylic) with a retronolone (2-alkenyl-3-methyl-cyclopent-2-en-4-olone), are insecticides of a natural orgin whose characteristics are a rapid and high insecticide action coupled with a low toxicity for mammals.
Pyrethrum is, however, easily degradable by the action of the atmospheric agents and this behavior makes it unsuited for the protection of agricultural cultivations, limiting its use only to indoor application. Moreover, it is rather expensive, partly because of the complexity of the extraction processes and partly due to the necessity to couple it with suitable synergic substances.
In order to overcome these problems, a great number of substances with a structure similar to that of pyrethrum (pyrethroids) have been synthesized with the aim of preserving the insecticide action and the low toxicity for mammals while at the same time obtaining molecules that are more resistant to the action of the atmospheric agents (see for instance "Synthetic Pyrethroids (M. Elliott Ed.): ACS Symposium Series No. 42, Washington 1977").
Research has been directed towards the synthesis of new derivatives of 2,2-dimethyl-cyclopropanecarboxylic acid and of analogs, as well as towards the introduction of new groups, carrying alcoholic functions, to be esterified with the derivatives or the analogs of the 2,2-dimethylcyclopropanecarboxylic acids.
In the following Table 1 there are recorded a few of the pyrethroids that have shown interesting properties.
TABLE I __________________________________________________________________________ ##STR6## Common Name Bibliographical W Z source __________________________________________________________________________ Pyrethrin I ##STR7## ##STR8## Natural origin Allethrin ##STR9## ##STR10## Schechter et al.J.A.C.S. 71. 3165 (1949) Rosmethrin ##STR11## ##STR12## Elliott et al., Nature (London), 213.493 (1967) Tetramethrin ##STR13## ##STR14## Kato et al., Agric.Biol. Chem. 28. 914 (1964) Phenothrin ##STR15## ##STR16## Fujimoto et al., Agric. Biol.Chem.37.2681 (1973) Permethrin ##STR17## ##STR18## M. Elliott et al., Nature (London); 246. 160 (1973) Pestic. Sci., 6.537 (1973) Cypermethrin ##STR19## ##STR20## M. Elliott - ACS Sympo- sium Series no 42 - Wash- ington 1977 Decamethrin ##STR21## ##STR22## M. Elliott - ACS Sympo- sium Series no 42 - Wash- ington 1977 __________________________________________________________________________